The present invention relates to a dual-axis resonator gyroscope and corresponding methods of operation.
Resonator (Coriolis) gyroscopes are well known, a comprehensive review of the current technology can be found in Huikai Xie and Gary Fedder “Integrated Micro electromechanical Gyroscopes”, Journal of Aerospace Engineering April 2003 and in U.S. Pat. No. 6,944,931.
The present invention deals with improvements upon the 2-axis gyroscope geometry illustrated in FIG. 15 of U.S. Pat. No. 5,763,781. This geometry is shown in FIG. 1—and hereinafter referred to as the reference geometry which includes four in-plane vibrating members 44, 45, 46, 47 that are narrowed at their bases and supported by a stationary anchor 48. The vibrating members are mutually coupled by means of flexure elements 49, 50, 51, 52 each of which has an in-plane resonant mode (also referred to as drive mode, primary mode, or excitation mode) and an out-of-plane resonant mode (also referred to as secondary mode, sense mode, or Coriolis mode). It should be emphasized that features of the present invention may be applicable to other geometries of vibratory gyroscopes.
As is well known to those skilled in the art Resonator gyroscopes can be operated either in an open loop mode or in a closed loop “force-balance” mode—as described further below.
In general the reference geometry suffers from several deficiencies:
1. Limited In-Plane Coupling Between Adjacent Vibrating Members
In-plane coupling is the ratio of static angular deflection induced in a one member to a deflection forced on an adjacent member. In-plane coupling is required in order to obtain equal amplitudes minimize the effects of production inaccuracies; it is limited to about 10% in the reference geometry.
2. Secondary Resonant Frequency Cannot be Flexibly Selected
It is well known that the sensitivity of a resonator gyroscope is maximized when the resonant frequencies of the drive and the Coriolis modes—depicted by arrows in FIG. 2, are nearly matched. The resonant frequency ω=√{square root over (k/I)} of a flexing beam, either in-plane or out-of-plane, depends on the respective moment of inertia I and stiffness k, where stiffness depends on the beam flexing length L, its width w, and its thickness h. Since the inertia of the vibrating members, as well as their flexing length, are nearly the same for both in-plane and out-of-plane vibration modes, the respective resonant frequencies cannot be brought to nearly the same value unless w nearly equals h—the wafer thickness. As is well known, in order to increase the sensitivity of a resonator gyroscope its excitation frequency should be relatively low, however the large h (thick wafer), required in order to increase the vibrating mass implies a respectively high resonance frequency.
3. Limited In-Plane Vibration Amplitude
In order to maximize the rate-induced Coriolis deflection the in-plane vibration amplitude of members 44, 45, 46, 47 should be as much as allowed by the material elastic limit and geometry. In general, the maximum deflection of a flexing element of length l is proportional to l3. In the reference geometry l is short compared to the physical length of the beam, thus permitting only limited vibration amplitude. Also, it was found that tangential coupling elements 49, 50, 51, and 52 as in FIG. 1 tend to break at relatively small amplitudes, because of stress concentration due to their short lengths.
4. Long Term Instability of the Matching Between the Two Resonant Frequencies
Stability of the scale factor—see below, and of the bias (zero-rate output) are important performance measures of any Resonator gyroscope, whether single-axis or dual-axis. Both are related to the degree of matching between the excitation frequency and secondary-mode resonant frequency over time and temperature. Some have considered such matching an insurmountable challenge—see for example: An Approach for Increasing Drive-Mode Bandwidth of MEMS Vibratory Gyroscopes, Cenk Acar and Andrei M. Shkel, JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 14, NO. 3, JUNE 2005
The various features of the present invention presented below address one or more of the deficiencies listed above and, in certain implementations, also provide:    1. A force balance loop that operates differentially on the Coriolis induced forces    2. A force balance loop that operates on the common mode out-of-plane deflection to resist Z-axis acceleration from affecting the rate measurement and provide Z-axis acceleration reading.    3. A force balance loop split into a “quadrature” and “in-phase” channels to eliminate the waste of electronic signal dynamic range by the quadrature signal and make it available for the useful signals.    4. An on-line self-test that transparently monitors the primary and secondary modes.